Recently, an apparatus to detect weak light has been widely introduced centering on a medical site and a research site. In such an apparatus, a photomultiplier tube which is relatively expensive is used as a detection unit of weak light.
Also, instead of the photomultiplier tube, an apparatus to detect weak light by using a complementary metal oxide semiconductor (CMOS) image sensor, which can be produced at a low price, is proposed (see, for example, Patent Document 1).